Specific gravity determination is one of the routine medical diagnostic tests carried out on several body fluids, most notably urine. In the case of urine, high specific gravity can be indicative of susceptibility to kidney stones, while low specific gravity can diagnose inability to concentrate urine which could underlie other kidney problems.
The specific gravity of urine is inversely related to the urinary output. Maintenance of high urinary output is a therapeutic goal necessary to avoid occurrence of kidney stone disease and/or urinary tract infections. High urinary output reflects sufficient water intake needed to prevent dehydration, a frequent condition in people living in warm climates, frequent airline travelers and in older individuals. Renal stone disease, which is directly related to high urine density, accounts for about 7-10 of every 1000 hospital admissions in the United States.
Normal urinary output varies between 0.25 to 5.0 liters a day depending on the amount of water intake. Urine density can vary accordingly between about 1.001 and 1.030. A urine density of 1.010-1.015 corresponds to an average desirable urinary output and water intake of 1-2 liters per day.
In theory, measurement of daily urinary output is simple. In practice, however, it is very difficult since it necessitates the collection of all the urine excreted during a precise 24 hour period. Thus, it is easier to measure the specific gravity of a urine sample.
Another use of frequent urine density (specific gravity) measurements is to monitor the kidney health of those drinking hard water or water with high salt contents. Hard water intake or prolonged drinking of water with high dissolved salt content increases the work load of the kidneys and can result in kidney stones. The finding of elevated urine specific gravity in those people necessitates replacing their regular water intake with low salt or even distilled water to lower the salt concentration and specific gravity of their urine and to reduce their potential for kidney stones and other kidney problems.
Specific gravity determinations are generally laborious and require the use of expensive and specialized equipment. The methods now available are not suitable for home use. The determination of urine specific gravity at home is needed to frequently monitor the water intake of patients on certain treatments or to follow up their conditions and/or the effectiveness of their treatments. Most importantly, specific gravity determinations are needed to monitor daily water intakes in kidney-stone-susceptible populations. Repeated urinary density measurements are necessary to ensure a permanent and persistent high daily urinary volume and water intake.